1. Technical Field
This invention relates generally to friction materials and more particularly to metal titanates, especially alkali metal titanates and alkali earth metal titanates, used for friction stabilization of friction materials employed in devices such as disk brake pads, brake drum linings, and other friction applications.
2. Discussion
Friction materials serve in a variety of ways to control the acceleration and deceleration of a variety of vehicles and machines such as, but not limited to, business equipment, bicycles, automobiles, and aircraft. A qualitative analysis of a friction couple suggests that a frictional force is likely to consist of several components such as adhesion tearing, ploughing (or abrasion), elastic and plastic deformation, and asperity interlocking, all occurring at the sliding interface. These mechanisms depend upon the temperature as well as the normal load and sliding speed, since material friction and wear properties are known to be dependent upon at least these variables. In the case of automotive friction materials, the coefficient of friction is usually found to decrease with increasing unit pressure and sliding speed at a given temperature. This decrease in friction is controlled by the composition and microstructure of friction materials.
For a fixed amount of braking, the amount of wear of automotive friction materials tends to increase slightly or remain practically constant with respect to brake temperatures, but once the brake rotor temperature reaches above 200.degree. C., the wear of resin-bonded materials increases substantially and sometimes unpredictably with increasing temperature. This wear is due in part to thermal degradation of organic components. At low temperatures, the practically constant wear rate is primarily controlled by abrasion and adhesion.
Wear is an economic consideration. Wear resistance generally is inversely related to friction and other desirable performance characteristics within any class of friction material. The formulator's objective is typically to provide the highest degree of wear resistance in the normal use temperature range, a controlled moderate increase at elevated temperatures, and a return to the original lower wear rate when temperatures again return to normal with materials that are cost effective. Maximum wear life does not require maximum physical hardness.
In an automotive vehicle, one of the purposes of the friction members is to physically stop the vehicle. These units, including disk brake pads and rotors and brake shoes and drums, convert physical movement into frictional heat. Every wheel contains either a disk or brake drum assembly.
Friction members for brakes are typically made from materials that will produce friction with the rotor or drum and withstand the high temperatures developed during braking. The most common type of friction materials used in brakes and clutches for normal duty is generally termed organic. These organic friction materials usually contain about 30-40 weight % of organic components. One of the main constituents of practically all organic friction materials at one time was asbestos fiber, although small quantities of other fibrous reinforcement may have been used. Since asbestos alone did not offer all of the desired properties, other materials called property modifiers, either abrasive or nonabrasive, were added to provide desired amounts of friction, wear, fade, recovery, noise, and rotor compatibility. A resin binder, such as phenolic or cresylic resin, held the other materials together in a matrix. This binder was not completely inert and made contributions to the frictional characteristics of the composite.
Still another type of friction material, semi-metallics, were introduced in the late 1960s and gained widespread usage in the mid 1970s. These materials usually contain more than 50 weight % metallic components. They are primarily used as friction materials for brake disk pads and blocks for heavy-duty operation. The main constituent of practically all semi-metallics is iron powder in conjunction with steel fiber. Semi-metallics generally contain no asbestos.
Because asbestos has been alleged to be the cause of certain health problems and is no longer environmentally acceptable, most modern brake pads and drum linings are made without asbestos. Thus, most current friction linings are made from synthetic and steel fibers, and iron, ceramic, and metallic powders.
Of recent interest is the use of low-metallic friction materials in disk pads, drum linings, and clutch disks. These materials generally contain less than 40 weight % metallic components. As a result, low-metallic friction materials are generally lighter than semi-metallic friction materials.
Certain titanium-based metal compounds have been previously employed in certain friction applications. These include potassium hexatitanate (K.sub.2 Ti.sub.6 O.sub.13) and potassium octatitanate (K.sub.2 Ti.sub.8 O.sub.17). These materials have typically been employed in the form of monoclinic crystal systems with a chemically and physically stable tunnel structure. These materials have been commercially available in primarily acicular single crystal (whisker) form, but the potassium hexatitanate has been made available from a melt in a "tabular" shape. Because these friction materials are generally comprised of "needle-like" fibers, it is generally believed that they may potentially produce similar health problems allegedly caused by asbestos.
There exists a continuous need for stable friction materials for use in disk brake pads, brake drum linings, and the like, to improve their overall performance.
It has now been discovered that titanate materials of the general formula --TiO.sub.3 are at least equally efficacious and also provide more desirable physical forms in a friction material environment.
The composition and methods of the present invention generally comprise at least two components:
(1) a titanium compound of the formula: PA1 (2) a friction material binder system, the binder system physically or chemically binding the titanium compound. The composition may optionally employ other conventional friction material constituents, such as a second binder, lubricants, fillers, metal fibers, and the like. PA1 at least one alkali metal titanate or alkali earth metal titanate of the general formula M.sub.A TiO.sub.3, where M is selected from the group consisting of alkali metals or alkali earth metals, and A is an integer of 1 or 2, the alkali metal titanate or alkali earth metal titanate being present in an amount of from about 3 volume percent to about 25 volume percent based on the total volume of the friction material matrix; and PA1 at least one binder system, the binder system physically or chemically binding the alkali metal titanate or alkali earth metal titanate. PA1 at least one alkali metal titanate or alkali earth metal titanate of the general formula M.sub.A TiO.sub.3, where M is selected from the group consisting of alkali metals or alkali earth metals, and A is an integer of 1 or 2; and PA1 at least one binder system, the binder system physically or chemically binding the alkali metal titanate or alkali earth metal titanate.
M.sub.A TiO.sub.3, where M is an alkali metal or alkali earth metal, and A is an integer of 1 or 2; and
Accordingly, it is at least one object of the present invention to provide a new and improved friction material.
It is another object of the present invention to provide a new and improved semi-metallic friction material.
It is another object of the present invention to provide a new and improved low-metallic friction material.
It is another object of the present invention to provide a new and improved friction material containing certain alkali metal titanates.
It is another object of the present invention to provide a new and improved friction material containing certain alkali earth metal titanates.
It is another object of the present invention to provide a new and improved friction material containing alkali or alkali earth metal titanates, wherein the alkali or alkali earth metal titanates are selected from the group consisting of sodium titanate, potassium titanate, calcium titanate, and mixtures thereof.
It is another object of the present invention to provide a new and improved braking element.
It is another object of the present invention to provide a new and improved automotive disk brake pad.
It is another object of the present invention to provide a new and improved automotive brake drum lining.
It is another object of the present invention to provide a new and improved automotive clutch disk.
In order to overcome the aforementioned disadvantages and achieve the aforementioned objects, the present invention provides a friction material matrix having improved friction stability and surface wear properties, in accordance with the following embodiments.